The present invention relates to an illumination device for illuminating a color liquid crystal display (LCD) as a front light.
An illumination device for the color LCD comprises a white LED and an illumination panel for illuminating the color LCD disposed below the illumination panel. In order to illuminate the LCD so that the LCD displays images with natural tone, it is necessary to balance the chromaticity of the white LED, the transmittance of the crystal in the LCD at every color, and the characteristic of a color filter provided in the LCD.
However, since it is difficult to adjust the transmittance of the crystal at every color, the chromaticity of the LED is adjusted.
FIG. 8 is a sectional view of a white LED. The white LED 50 comprises a substrate 51, a blue LED element 52 on the substrate 51 and a YAG fluorescent layer 53 covering the LED element 52. Blue light from the LED element 52 excites the YAG fluorescent layer to cause it to emit light yellow light. The blue light and light yellow light are mixed to produce white light.
FIGS. 9 through 13 are graphs showing spectrums of the light emitted from the white LED 50. The horizontal axis is the wave length (nm), the vertical axis is the relative intensity of the spectrum. In the graph, the red light has a wave length of about 625 nm, green light has a wave length of about 560 nm, and blue light has a wave length of about 450 nm.
FIG. 9 shows a spectrum H1 of normal white light emitted from the LED 50. The spectrums E2 and H3 of FIGS. 10 and 11 show lights of slightly blue, and H4 and H5 of FIGS. 12 and 13 show lights of slightly yellow.
FIG. 14 is a CIE chromaticity diagram showing the proportion of the chromaticity of the light emitted from the white LED. The X-axis shows the proportion of red, Y-axis shows the proportion of green and Z-axis shows the proportion of blue. There is a relationship of X+Y+Z=1. The point CO at a central position is a point of the proportion of chromaticity of 1:1:1. The coordinate of the point CO is X=0.33, Y=0.33 and Z=0.33. An area S surrounded by a dotted line is regarded as a white color range.
In order to provide a color LCD having a natural tone, it is necessary to provide LEDs with various chromaticities. However, the preparation of various LEDs increases the manufacturing cost.